1. Field of the Invention
This invention relates to thermoplastic molding compositions based on polyoxymethylene blends and more particularly to blends of polyoxymethylene polymers with small amounts of selected low molecular weight copolyether-ester resins to give polyoxymethylene polymer blends with improved molding properties.
2. Relation to the Prior Art
Thermoplastic molding resins based on polyoxymethylene blends are well-known in the art. For example, U.S. Pat. No. 3,795,715, describes mixtures of oxymethylene polymers with polyesters made from linear aliphatic dicarboxylic acids and .alpha., .omega.-diols; the addition of the polyester to the polyoxymethylene improves its toughness. Similarly, it is known that the properties of polyester molding compositions, based on linear saturated polyesters of aromatic dicarboxylic acids, can be improved by the addition of 0.1-25% of a polyacetal (U.S. Pat. No. 3,595,937). Polyoxymethylene polymers which contain up to 15% of a polyalkylene terephthalate have a more uniform spherulite size and improved color stability (U.S. Pat. No. 3,696,171), and polyoxymethylene/aliphatic polyester block polymers are known (U.S. Pat. No. 3,580,973). Oxymethylene polymer compositions with improved impact resistance are prepared by incorporating a rubbery polymeric material which contains pendant nitrile or carboxylic ester group (U.S. Pat. No. 3,476,832; Canadian Patent No. 829,368. Incorporation of up to 10% of selected polymeric materials with T.sub.g &lt; 30.degree. C, including polyethylene oxide and polytetrahydrofuran, gives polyoxymethylene molding compositions of improved strength Canadian Patent No. 844,683; U.S. Pat. No. 3,795,715). Such polymers would be expected to exhibit improved molding properties, but polymer blending is difficult because of the low melting points and hygroscopic nature of the polyethers.
More recently, published German Patent No. 2,449,343, describes molding resins with improved impact resistance which are blends of an oxymethylene polymer with 0.1-50 weight % of a high molecular weight copolyester consisting of 75-35% of long chain ester units which contain ether linkages and 25-65% of short chain ester units. A plot of the reduced specific viscosity (RSV), measured as described in German Patent No. 2,449,343 vs. the inherent viscosity in m-cresol shows that polymers disclosed in the reference have inherent viscosities above 0.95, namely 1.0 and 1.7.